Influenza A H1N1 represents a major cause of morbidity and mortality in the United States. In addition, Influenza A poses a significant risk of pandemic outbreak as evidenced in the past two years. A large proportion of severe cases of Influenza A pneumonia are associated with secondary bacterial infection, most commonly caused by Staphylococcus aureus or Streptococcus pneumoniae. Incidence and severity of S. aureus pneumonia is increasing worldwide due to the emergence of methicillin-resistant (MRSA) strains. For these reasons, understanding the molecular mechanisms that promote bacterial host defense in the lung is of critical importance. Little is known about the cell-mediated immune response to S. aureus infection. Using a mouse model of Influenza A infection (Influenza A PR/8/34) coupled with S. aureus challenge our group has found that Influenza A exacerbates secondary bacterial pneumonia. We have shown that the mechanism is likely mediated by type I Interferon suppression of IL-23 production and subsequent TH17 immune activation. In this proposal we will further investigate the Influenza A, S. aureus co-infection model in three specific aims. First, we will examine the role of type I interferon in mediating Influenza A exacerbation of secondary bacterial infection. Second, we will examine the mechanism by which IL-23 promotes immunity against S. aureus. Finally, we will investigate the mechanism by which the TH17 pathway promotes S. aureus killing via the airway epithelium. These studies will involve numerous TH17 pathway gene altered mouse studies and in vitro studies with both mouse and human airway epithelial cells. The goal of the study is to elucidate the molecular mechanisms involved in S. aureus host defense and to identify interventions that can restore TH17 immunity following Influenza A infection and improve the host response against secondary bacterial pneumonia. These data may be directly applicable to the hospital setting and may reveal novel therapeutic strategies that would decrease morbidity and mortality, and improve patient outcome.